1. Field of the Invention
The present invention concerns a device for the acquisition of the angular position of moving mechanical means, and also opto-mechanical systems incorporating such devices.
The present invention is particularly designed for the production of thermal scanning cameras. It is not, however, limited to such an application, and is suitable for use in many opto-mechanical systems such as scanners, imagery systems, laser projection systems or servomechanisms.
2. Description of the Related Art
Particularly known in the prior art are the thermal cameras designed by the applicant, implementing devices for position sensing and automatic control of the scanning mirror. These devices make it possible to accurately control the orientation of a field mirror executing a saw tooth movement. The position sensing accuracy particularly affects the quality of the image and the performance of the equipment, as a modification of the scanning amplitude or a defect in the linearity of the rotational movement can cause an anamorphosis or lead to unacceptable deformations of the image.
The position sensor must also detect any drift of the optical axis of the scanning mirror, in particular when the thermal camera is used in position finding and target sighting for the firing and guidance of missiles.
In the prior art, the system of acquisition of the instantaneous position of a scanning mirror makes use of one angular copy sensor and a second reference copy sensor.
The first sensor includes a luminous source comprising a light emitting diode, collimation optics, a diaphragm of special shape capable of linearizing the response of the sensor, a mirror fixed to the back of the scanning mirror and a photosensitive sensor with two complementary quadrants. The beam, collimated to infinity, hits the two quadrants after being reflected by the mirror attached to the back of the scanning mirror.
The light flux received by each of the two complementary quadrants varies according to the angular position of the scanning mirror. In fact, when the mirror is in a median position, each of the quadrants receives an identical flux. When the mirror is offset from its median position, one quadrant receives the flux on part of its surface only, whereas the other quadrant receives a flux on the whole of its surface. The output voltage of the first quadrant is consequently lower than that of the complementary quadrant, and an appropriate analog circuit delivers a signal corresponding to the difference between the output voltages.
Signal drift caused by variations of the emission power are corrected by regulating the source power depending on the sum of the voltages delivered by the two cells. The geometrical shape of the quadrants is particularly important, because any variation would lead to imperfect linearity of the photosensitive sensor response. Moreover, it is clear that the size of each of the quadrants must be approximately half that of the course scanned by the reflected beam, which means that photoresistant cells must be used in the manufacture of each of the quadrants. Given the relatively large size of this photosensitive sensor, it is difficult to prevent drift owing to the different response curves of the two quadrants and the thermal fluctuations experienced by these quadrants.
Moreover, in order to facilitate operation of the device over the whole course of the mobile mirror, the beam must have substantially the same width as the sensor. Since known sensors operate according to a principle of differential measurement of the flux, it is essential that, regardless of the position of the reflected beam, each quadrant receives part of this beam. The luminous source must consequently have sufficient intensity to make the flux of the reflected beam compatible with a correct response of the two photoresistant quadrants, even when only a small part of the quadrant is illuminated, which occurs when the moving means is in an end position. For this reason, the energy consumption of the previously known system is considerable.
Furthermore, in order to compensate for drift of the first sensor, it is necessary to have a second sensor, the purpose of which is to memorize the instantaneous value of the signal output by the first sensor, when the mirror is in a reference position. This voltage is then fed back into the control loop in order to compensate any possible drift.
Previously known devices present various disadvantages. In particular, the known sensing system requires very precise mechanical adjustment of the different components, and is for this reason relatively sensitive to thermal variations. Furthermore, it requires specific expensive components, such as selected and mated photosensitive sensors, or specific electroluminescent diodes. In addition, in order to improve the precision of the second sensor, multiple reflections are used to create an optical lever arm. This solution involves adjusting and setting difficulties.
Other solutions implement inductive or potentiometric sensors. These solutions, however, are not always completely satisfactory, notably because they complicate the scanning mirror driving systems and moreover because they do not provide sufficient precision.